Method and apparatus for reducing wrinkle formation in deep drawing

Abstract

A method and apparatus for deep drawing a product from a blank ( 5 ), in which, during deep drawing, the blank ( 5 ) is held near its edge by a downholder ( 8 ) against a die ring cooperating with the downholder ( 8 ), thereby preventing, at least reducing wrinkle formation in the blank, by adjusting, at the beginning of deep drawing, the downholder ( 8 ) such that a downholder ( 8 ) force exerted by the downholder ( 8 ) on the edge of the blank is relatively small, and by controlling, during the further course of the process, the downholder ( 8 ) on the basis of a predetermined thickness trend of the mentioned edge during deep drawing and/or a trend or critical value derived from this thickness trend.

1 . A method for deep drawing a product from a blank, in which, during deep drawing, the blank is held near its edge by a downholder against a die ring cooperating with the downholder, thereby preventing, at least reducing wrinkle formation in the blank, by controlling the downholder, characterized in that at the beginning of deep drawing the downholder ( 8 ) is adjusted such that a downholder force (F N ) exerted by the downholder ( 8 ) on the edge ( 6 ) of the blank ( 5 ) is relatively small and the further control of the downholder ( 3 ) occurs on the basis of a predetermined thickness trend of said edge ( 6 ) during deep drawing and/or a trend or critical value derived from this thickness trend.  
   
   
       2 . A method according to  claim 1 , characterized in that during deep drawing a parameter relevant to the wrinkle formation is measured and compared with a critical value at which wrinkle formation occurs, which critical value is predetermined, based on the thickness trend of the blank edge ( 6 ) and/or a signal derived from this thickness trend, and in which, when the measured parameter exceeds or threatens to exceed this critical value, the downholder ( 8 ) is controlled such that a downholder force (F N ) exerted on the edge ( 6 ) by the downholder ( 8 ) increases and the measured parameter falls below the critical value.  
   
   
       3 . A method according to  claim 2 , characterized in that the parameter relevant to wrinkle formation is the downholder opening (s o ), defined as the perpendicular distance between the downholder ( 8 ) and the die ring ( 10 ), and the critical value is the predetermined thickness trend or the predetermined maximum thickness increase of the edge ( 6 ).  
   
   
       4 . A method according to  claim 2 , characterized in that the parameter relevant to wrinkle formation is the speed (v o ) at which the downholder opening changes, and the critical value is the predetermined speed trend or the predetermined maximum speed at which the blank edge increases in thickness during deep drawing.  
   
   
       5 . A method according to any one of claims  2 - 4 , characterized in that the critical value is measured during a testing session, previous to deep drawing.  
   
   
       6 . A method according to any one of claims  2 - 4 , characterized in that the critical value is simulated by means of a dynamic model of the blank ( 5 ) and the deep drawing process.  
   
   
       7 . A method according to  claim 1 , in which the position of the downholder ( 8 ) is controlled according to a predetermined range, such that during deep drawing the downholder opening (s o ), defined as the perpendicular distance between the downholder ( 8 ) and the die ring ( 10 ) substantially corresponds with a predetermined thickness trend of the edge ( 6 ) to be expected during deep drawing.  
   
   
       8 . An apparatus for deep drawing a product from a blank ( 5 ), comprising a downholder ( 8 ), a die ring ( 10 ) cooperating therewith, for holding an edge ( 6 ) of the blank ( 5 ) during deep drawing, a control ( 12 ) provided with means for storing therein a desired downholder opening trend (s o ), downholder opening speed trend (v o ) and/or a critical value derived therefrom, and positioning means ( 9 ) for moving the downholder ( 8 ), the control ( 12 ) being arranged to control the positioning means ( 9 ) such that the movement of the downholder ( 8 ) is in agreement with the stored downholder opening trend (s o ), downholder opening speed trend (v o ) and/or a critical value derived therefrom.  
   
   
       9 . An apparatus according to  claim 8 , in which the control ( 12 ) is arranged to control the positioning means ( 9 ) such that a distance between the downholder ( 8 ) and the die ring ( 10 ) is in agreement with the stored downholder opening trend (s o ) or a critical value derived therefrom.  
   
   
       10 . An apparatus according to  claim 8  or  9 , in which measuring means ( 11 ) are provided to measure the downholder opening trend (s o ), the downholder opening speed trend (v o ) and/or a critical value derived therefrom, and in which the control ( 12 ) comprises means for comparing signals measured with the measuring means ( 11 ) with a downholder opening trend (s o ), downholder opening speed trend (v o ), stored in the control ( 12 ) and/or a critical value derived therefrom, and in which the control ( 12 ) is arranged to control, on the basis thereof, the downholder ( 8 ) such that the movement, speed or critical value of the downholder ( 8 ) derived therefrom is in agreement with the stored downholder opening trend, downholder opening speed trend and/or the critical value derived therefrom.  
   
   
       11 . A apparatus according to any one of claims  8 - 10 , characterized in that the measuring means ( 11 ) comprise a contactless sensor, for instance an optical, capacitive or magnetic sensor.